Field of the Invention
The present invention relates to gear lubricant formulations.
Introduction
In industrial and automotive gears and in automotive engines, lubricants are used to minimize wear and friction between contacting surfaces. In some contact zones, such as meshing gear teeth, very high contact pressures are experienced. In some cases the heat generated from high friction can cause welding of the contacting surfaces. In order to protect equipment on high contact pressure applications lubricants are often formulated with sulfur-containing extreme pressure (EP) additives. Sulfur-containing EP additives react with a metal surface in the high temperature contact zone and form a thin tribo film of iron sulfide or other organometallic complexes that are rich in iron and sulfur, which rapidly form and deplete, protecting the metal surface from degrading. The sulfur content resulting from the EP additives in industrial lubricants can be as high as 15,000 weight parts per million (ppm) and in automotive gear oil lubricants the sulfur content can be as high as 25,000 ppm.
Unfortunately, the presence of sulfur in lubricant formulations can present challenges. For instance, sulfur containing EP additives can degrade to form compounds that lead to varnish and sludge in high temperature applications, thereby reducing the life of the equipment it is lubricating. Sulfur is also corrosive towards yellow metals (for example, copper alloys) so lubricant formulations used in yellow metal environments require additional corrosion inhibitor and sulfur scavengers to meet corrosion resistant requirements.
It is desirable to identify a way to reduce the amount of sulfur EP additive in a lubricant formulation without reducing the extreme pressure performance of the lubricant formulation.